Adenosine kinase (ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) is a ubiquitous enzyme which catalyzes the phosphorylation of adenosine to AMP, using ATP, preferentially, as the phosphate source. Magnesium is also required for the reaction, and the true cosubstrate is probably the MgATP.sup.2- complex (Palella, et al., J. Biol. Chem. 1980, 255: 5264-5269). Adenosine kinase has broad tissue and species distribution, and has been isolated from yeast (Leibach, et al., Hoppe-Seyler's Z. Physiol. Chem. 1971, 352: 328-344), a variety of mammalian sources (e.g. Miller, et al., J. Biol. Chem. 1979, 254: 2339-2345; Palella, et al., J. Biol. Chem. 1980, 255: 5264-5269; Yamada, et al., Comp. Biochem. Physiol. 1982, 71B: 367-372; Rottlan and Miras-Portugal, Eur. J. Biochem., 1985, 151: 365-371) and certain microorganisms (e.g. Lobelle-Rich and Reeves, Am. J. Trop. Med. Hyg. 1983, 32: 976-971); Datta, et al., J. Biol. Chem. 1987, 262: 5515-5521). It has been found to be present in virtually every human tissue assayed including kidney, liver, brain, spleen, placenta and pancreas (Andres and Fox, J. Biol. Chem. 1979, 254: 11388-11393).
Adenosine kinase is a key enzyme in the control of the cellular concentrations of adenosine (Arch and Newsholm, Essays Biochem. 1978, 14: 82-123). Adenosine is a purine nucleoside that is an intermediate in the pathways of purine nudeotide degradation and salvage. In addition, adenosine has many important physiologic effects, many of which are mediated through the activation of specific ectocellular receptors, termed P.sub.1 receptors (Burnstock, in Cell Membrane Receptors for Drugs and Hormones, 1978, (Bolis and Straub, eds) Raven, New York, pp. 107-118; Fredholm, et al., Pharmacol. Rev. 1994, 46: 143-156). In the central nervous system, adenosine inhibits the release of certain neurotransmitters (Corradetti, et al., Eur. J. Pharmacol. 1984, 104: 19-26), stabilizes membrane potential (Rudolphi, et al., Cerebrovasc. Brain Metab. Rev. 1992, 4: 346-360), functions as an endogenous anticonvulsant (Dragunow, Trends Pharmacol. Sci. 1986, 7: 128-130) and may have a role as an endogenous neuroprotective agent (Rudolphi, et al., Trends Pharmacol. Sci. 1992, 13: 439-445). Adenosine has also been implicated in modulating transmission in pain pathways in the spinal cord (Sawynok, et al., Br. J. Pharmacol. 1986, 88: 923-930), and in mediating the analgesic effects of morphine (Sweeney, et al., J. Pharmacol. Exp. Ther. 1987, 243: 657-665). In the immune system, adenosine inhibits certain neutrophil functions and exhibits anti-inflammatory effects (Cronstein, J. Appl. Physiol. 1994, 76: 5-13). Adenosine also exerts a variety of effects on the cardiovascular system, including vasodilation, impairment of atrioventricular conduction and endogenous cardioprotection in myocardial ischemia and reperfusion (Mullane and Williams, in Adenosine and Adenofine Receptors, 1990 (Williams, ed) Humana Press, New Jersey, pp. 289-334). The widespread actions of adenosine also include effects on the renal, respiratory, gastrointestinal and reproductive systems, as well as on blood cells and adipocytes.
Endogenous adenosine release appears to have a role as a natural defense mechanism in various pathophysiologic conditions, including cerebral and myocardial ischemia, seizures, pain, inflammation and sepsis. While adenosine is normally present at low levels in the extracellular space, its release is locally enhanced at the site(s) of excessive cellular activity, trauma or metabolic stress. Once in the extracellular space, adenosine activates specific extracellular receptors to elicit a variety of responses which tend to restore cellular function towards normal (Bruns, Nucleosides Nucleotides, 1991, 10: 931-943; Miller and Hsu, J. Neurotrauma, 1992, 9: S563-S577). Adenosine has a half-life measured in seconds in extracellular fluids (Moser, et al., Am. J. Physiol. 1989, 25: C799-C806), and its endogenous actions are therefore highly localized.
The inhibition of adenosine kinase can result in augmentation of the local adenosine concentrations at foci of tissue injury, further enhancing cytoprotection. This effect is likely to be most pronounced at tissue sites where trauma results in increased adenosine production, thereby minimizing systemic toxicities. Pharmacologic compounds directed towards adenosine kinase inhibition provide potentially effective new therapies for disorders benefited by the site- and event-specific potentiation of adenosine.
A number of compounds have been reported to inhibit adenosine kinase. The most potent of these include 5'-amino,5'-deoxyadenosine (Miller, et al., J. Biol. Chem. 1979, 254: 2339-2345), 5-iodotubercidin (Wotring and Townsend, Cancer Res. 1979, 39: 3018-3023) and 5'-deoxy-5-iodotubercidin (Davies, et al., Biochem. Pharmacol. 1984, 33: 347-355).
Adenosine kinase is also responsible for the activation of many pharmacologically active nucleosides (Miller, et al., J. Biol. Chem. 1979, 254: 2339-2345), including tubercidin, formycin, ribavirin, pyrazofurin and 6-(methylmercapto)purine riboside. These purine iucdeoside analogs represent an important group of antimetabolites which possess cytotoxic, anticancer and antiviral properties. They serve as substrates for adenosine kinase and are phosphorylated by the enzyme to generate the active form. The loss of adenosine kinase activity has been implicated as a mechanism of cellular resistance to the pharmacological effects of these nucleoside analogs (e.g. Bennett, et al., Mol. Pharmacol., 1966, 2: 432-443; Caldwell, et al., Can. J. Biochem., 1967, 45: 735-744; Suttle, et al., Europ. J. Cancer, 1981, 17: 43-51). Decreased cellular levels of adenosine kinase have also been associated with resistance to the toxic effects of 2'-deoxyadenosine (Hershfield and Kredich, Proc. Natl. Acad. Sci. USA, 1980, 77: 4292-4296). The accumulation of deoxyadenosine triphosphate (dATP), derived from the phosphorylation of 2'-deoxyadenosine, has been suggested as a toxic mechanism in the immune defect associated with inheritable adenosine deaminase deficiency (Kredich and Hershfield, in The Metabolic Basis of Inherited Diseases, 1989 (Scriver, et al., eds), McGraw-Hill, New York, pp. 1045-1075).